If a unit is killed in combat, since Endless Riches says to banish the unit, does that count as me banishing the unit for Zed Legend?

Yes, it counts as you banishing the card.

When Endless Riches' replacement effect turns a death into a banishment, you (the controller of Endless Riches) are the one performing the banish. Rule 374 states that a replacement effect's controller is the player who controls its source. Endless Riches says "banish it instead" — that banish action originates from your permanent, so you are doing the banishing.

Since the unit being banished is a card you own (it entered play from your deck/hand), both conditions on Master of Shadows — "When you banish a card you own" — are satisfied. Master of Shadows will empower.

This is consistent with how other replacement effects work. For example, when Zhonya's Hourglass replaces a death with "heal that unit, exhaust it, and recall it," those actions (heal/exhaust/recall) are attributed to Zhonya's controller, not the original source of the damage (Rule 370.2 example). Same logic applies here — the banishment is attributed to you as the controller of Endless Riches.